Heat resistance (resistance to high temperature gases or vapor) is the most important property of heat-resistant steels. As a rule, heat-resistant steels must also be heat-resistant, i.e. they must resist creep - gradual and increasing deformation over time under constant load, leading to failure - at high temperature for a specified time.

Hardening of heat-resistant steels

In addition to chromium carbides, hardening phases include vanadium, molybdenum, tungsten carbides and intermetallic compounds. elements, as well as intermetallic compounds of the AgB type (which include iron and chromium as element A and molybdenum, tungsten, niobium, titanium as element B) or compounds (Ti, Al). Additions of refractory elements - molybdenum, tungsten, niobium, tantalum - to refractory steel have a stabilizing effect on the hardening phase, because these elements increase the recrystallization temperature and weaken the diffusion processes. Their effect is enhanced when more than one diffusion-weakening element is introduced. For this reason, heat-resistant steels are alloyed, as a rule, with a set of different elements.

Types of heat-resistant steels

Diffusion exchange processes can also be retarded if the steel does not undergo polymorphic transformation. That is why purely ferritic or austenitic steels that are complex alloyed are often used as heat-resistant steels. Until recently, ferritic steels were only used as heat-resistant steels. However, heat resistant ferritic steels, for example 12X2MV8FB (EP503), hardened with intermetallic phase FeW particles, have recently been developed and are being successfully introduced. Austenitic steels contain 12-20% Cr and are characterized by much higher heat resistance. Austenitic steels with 7-30% Ni as an austenitizing element are particularly widely used. Nickel itself is a corrosion-resistant metal and increases the corrosion resistance of steels in solutions of salts and alkalis, as well as in weakly acidic environments. At its content of up to 20-30%, it increases the heat resistance of iron-chromium alloys. Because of the high cost of nickel in some heat-resistant steels, it is partially or completely replaced by another austenitic element - manganese. Its effect as an austenitic element is much weaker, especially when the content of chromium is high, so together with Mn reasonable to enter a small amount of nickel 2-4% or nitrogen. Carbon additives with vanadium, molybdenum, tungsten, niobium and nitrogen are recommended for high heat resistance.

Intergranular corrosion

Chromium-nickel, chromium-nickel-manganese and chromium-manganese heat-resistant steels resist general corrosion well but are sensitive to intergranular corrosion, especially after slow cooling in the temperature range of 500-850°C. This is explained by the release of chromium carbides at these temperatures at grain boundaries. In electrolyte solutions the carbides form galvanic pairs with carbon-depleted grain areas. As a result of the structural heterogeneity, the grain boundaries are subjected to more severe corrosion erosion. Austenitic steels become insensitive to intergranular corrosion if the carbon content of the steel is less than the limit of its solubility in austenite at room temperature, i.e. less than 0.02-0.03%.

Production

It is difficult to smelt heat resistant steel with such a low carbon content in electric arc furnaces. Therefore smelting of corrosion-resistant austenitic steels the upper limit of carbon content is usually set at 0,08-0,12% and further reduction of carbon concentration in the solution is performed by additives of strong carbide-forming elements - titanium or niobium. The amount of titanium introduced is determined by the carbon content, and for sufficiently complete carbon binding the amount of titanium must be at least five times the amount of carbon. The high chromium and titanium content in this type of steel leads to intense oxidation of the metal during casting with the formation of a crust on the surface of the metal in the mold, rich in oxides and nitrides of titanium. Crust twists when filling the mold lead to numerous surface defects of heat-resistant steel ingots, which force the ingot to undergo a continuous skinning to a depth of 10-20 mm.
Clusters of nitrides and oxides remaining in the ingot body form a marginal or general heterogeneity of macrostructure - the so-called titanium porosity. The degree of development of this defect in refractory steel increases with increasing titanium content in the metal.

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